How many Apple IDs should one household have?

Question

To date my wife and I share one Apple ID for all Apple purchases and devices. We currently own a first and third generation iPod, iPhone 4, and will own an iPad 2 in the morning. Thus far having the one account has not been an issue, but with more Apple purchases to follow at what point does it make sense to have more than one Apple ID? What are the advantages and disadvantages of having one or multiple accounts? If we each had an iPhone would multiple Apple IDs be required? What's the relation to iCloud?